Hydrostatic transmissions use fluid pressure to transmit power from a power source to a power output. Hydrostatic transmissions may be used for a variety of heavy-duty applications such as heavy-lifting equipment and agricultural or earth-moving machines. The basic unit of a hydrostatic transmission is a hydrostatic pump and a hydrostatic motor which may be connected to each other through a closed hydraulic loop that carries high pressure fluid between the pump and the motor. The hydrostatic pump may be connected to the engine crankshaft, and may convert the mechanical rotary power from the engine into hydraulic power which is transmitted to the hydrostatic motor. The hydrostatic motor may then convert the hydraulic power into mechanical rotary power for transfer to the wheels/tracks or other power output of the machine.
The amount of fluid flowing through the hydrostatic transmission may be controlled by adjustment of a tilt angle of a swashplate associated with either or both of the hydrostatic pump and motor. The tilt angle of the swashplate may control the amount and direction of fluid flow through the system, thereby controlling the speed and direction (forward or reverse) of the transmission. In particular, when the swashplate is vertical (or has a zero tilt angle), fluid does not flow through the system and the transmission may be in neutral. When the swashplate is tilted at a forward angle, a forward direction of the transmission is provided, and when tilted backwards, a reverse direction is provided. As the forward (or backward) tilt angle of the swashplate increases, the forward (or reverse) speed of the transmission increases.
Thus, hydrostatic transmissions are continuously variable transmissions that provide infinite speed and torque changes from full forward to full reverse by adjustment of the swashplate tilt angle without the need for gear shifting. In addition, hydrostatic transmissions permit the engine to operate at a constant speed even when the machine load or machine speed is changing. One disadvantage of hydrostatic transmissions, however, is that they may have lower efficiencies at high operating speeds. In particular, under high speed conditions, the hydrostatic motor operates at high speeds and uses more engine power. This is in contrast with direct drive transmissions (transmissions in which power from the engine is transmitted to the output axle through various gearing) which may be 15-20% more efficient than pure hydrostatic transmissions at high operating speeds.
In efforts to improve the efficiencies of machines relying on hydrostatic transmissions, systems that shift from hydrostatic transmission at low speed conditions to direct drive transmission at high speed conditions have been described (see, for example, U.S. Pat. No. 6,139,458 and WO 2014/122322 A1). While effective, alternative transmission arrangements that accommodate varying machine operating conditions and power or other requirements are still wanting. The present disclosure addresses these and other problems of the prior art.